Electrical transmission circuit



Aug. 19, 1941..

2 Sheets-Sheet l Filed Jan. 27, 1940 :ET 1 l @a LH v- \b\n Nb uw W m m mw m W a W 3 w w r m mm n .et m mw e .Tiff W7 al @www a if m1 Q M NIN lFil im M mw l M MT-HIS IP \Q\ y fha., %NN-* II M .o L Wi Aug'. 19, 1941.v w. A. MacNAlR 2,253,0791f i ELECTRICAL TRANSMISSION CIRCUIT Filed Jan.27, .1940. 2 sheets-sheet?" arrow/Ev"v means Patented Aug. 19, 1941UNITED STATES, PATENT ostias ELECTRICAL TRANSMISSION CIRCUIT Walter A.MacNair, Summit, N. J.,- anssignor to'Bel'l Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York pplica'tionJa'nuary 27, 1940,Serial No. 316,055 15 Claims. (Cl. 179-1003) This invention relates toelectrical transmission gs'yetems and particularly to the transmission`circuit of a sound reproducing system.

The object of the invention is to Connect two sources of' similarCurrents which may be either in phase 'conjunction or in phaseopposition, automatically to a single utilization circuit so that thecurrent in the utilization circuit will be in'phase conjunction.A t

A feature of the invention is the provision of for detecting the phaserelationship of the currents Vfrom the sources. Another rfeature of theinvention is the provision of means automatically responsive to theplrase1 relationship of the currents from the sources to connect bothsourcesto a single utilization circuit with the `currents from bothsources in phase conjunction.

Other features of the invention will be appa- `rent `'from the followingdescription and the drawings.

The invention is applicable to any transmission system having twosources of current, which may loe in phase conjunction or phaseopposition, `but is peculiarly applicable to sound reproducing systems.In two of the known methods of sound recording, particularly therecording of sound on a photographic iilm, the sound iS recorded 'eitheron a single record, or on two records in push-pull relationship. Inconnection with the present invention, the single sound record may `beconsidered as two records having the sound recorded in phaseconjunction, that is, the two reco-rds are in phase, and the push-pullrecord maybe considered as two records in which the sounds are recordedin Aphase opposition, `that is, `the two records are 180 degrees out ofphase witheach other or in opposed phase. Separate piek-ups orreproducers are used for the two parts of the normal, or in phase,record land l`e`p`a`rate `pick-ups are also used for the two parts ofthe` pushepull, or opposed phase, record. These two pick-ups thus act astwo sources of ncurrents which may be in phase conjunction or phasepposition. A circuit in accordance with 4the present invention isconnected between the two pick-upsand the usual single channelreproducing circuit. The operator then may play either `type of record,or even a single production in which both types of records are usedindiscriminately, and the `invention will automatically detect the typeof record being played and will modify the reproducing circuit so thatin al1 `cases the currentsfrom the two sources are supplied to thesingle reproducing channelin phase conjunction.

The invention is disclosed in greater detail in vthe followingdescription, and in the drawings, in

which: ,p

Fig. 1 showsan lembodiment of the invention using two double-wondtransformers and separate detectors; Y Fig. 2 showsa'n embodiment of theinvention using two double-wound transformers and a single detectoriFig; 3 shows an embodiment of the invention using a single double-woundtransformer and a single detector in the mid-branch;

"Figjll shows a modification of'Fig. l; and

Fig. 5`shows an embdiment of the invention using a single detectora'nd abridged network.

` In order to conveniently demonstrate the operation of` a circuitembodying the invention, a special input circuit has been used inconnection with Fig. 1. i Current from some' convenient source, such asa conventional pick-up repro ducing a known type of sound record, isapplied Aacross the terminals of the resistor I. The primary winding uofthe transformer 2 is permanently connected across the terminals of theresistor I, thus the currents in the primary winding of transformer 2are in phase with the `voltage developed across the resistor I. With'the key I0@ in the unoperated position,` as shown, the primary windingof the transformer 3 is connected to the resistor I in the oppositesense to the connection of the primary winding of transformer 2 to theresistor I, thus the currents in the secondarywindings of transformers 2and 3 .are in phaseop'position When the key IMI is operated, `theprimary winding of the transformers 2 and 3 are connected yto theresistor I in the same sense, thus, in this case, the currents inducedin the primary :windings of the transformers 2 and 3 will b'e in phaseconjunction. The input circuit Comprising the resistor I and the key IM,thus provides a convenient `method of observing the operation of thecircuit, as the key |00 may be repeatedly operated and released duringthe reproduction of a single record. In the normal operation of thecircuit, one pick-up would be attached to the primary winding of thetransformer 2 and a separate pick-up `would be attached to theV primarywinding of the transformer 3. The output of transformer 2, suitablyampliiied if desired in the amplifier 4, `is`supplied to the primarywinding of transformer 6. Similarly the output of transformer -3,suitably amplified if desired in the ampliiier `sired utilizationcircuit such as a 5, is' supplied to the primary winding of thetransformer 1. The transformers 6 and 'l each have double secondarywindings. The secondary winding 8 of the transformer 6 is connected inaiding relationship with the secondary winding 970i the transformer 'Iand the combined output of these windings is supplied to the transformerI2. The secondary winding I 9 of the transformer 6 is connected inopposing relationship to the secondary winding I I of the transformer Iand the combined output of these windings is supplied to the primarywinding of the transformer I3. The output of transformer |12 is suppliedto the thermionic detector I6 having a battery I8 to cause gridrectification and supplying operating current to the winding of therelay 23. Similarly the output of the transformer I3 is supplied to thethermionic detector Ai'ihav'- ing a battery I9 for biasing the grid,and. supplying operating current to the winding of the relay 2|. Theresistors I4 and I5 are respectively connected across the primarywindings of the transformers I2 and i3 to improve the impedancerelationships in the circuit. The operation of'relay 29 completes acircuit for the battery 24 through-the winding of relay 22 and operatesrelay, 22.

Similarly the operation fof relay 2l completes a circuit for battery 2 5through the windingof relay 23 and operates relay 23.

With the switch `Illil in the position shown, that is, the currents inthe secondary windings of transformers 2 and 3 and in the windings oftransformers 3 and 1 in phase opposition, the currents in the windings 8and 9 will be in phase opposition and will annul each other, thus nocurrent is applied tothe winding of transformer I2, but the currents inthe secondary windings I3 and II will assist each other and will supplycurrent through the transformer I3 to Vbe detected vto the thermionicdetector I'I and i operate relay 2I, thus operating rel-ay 23. Theoperation of relay 23 completes a circuit from the lower end of windingII through the righthand make contacts of relay 23 to the transmissionline 26. The lower end of winding I il is connected through ground tothe transmis- Asion line 21, thus the windings I Iand Il are effectivelyconnected across the transmission line 26, 21 and the outputs oftransformers 6 and 'I, properly in phase with each other, are suppliedto the transmission line 26, 2l. The transmission line 26, 2'! may beconnected to any deregular single channel sound reproducing equipment.

With the switch |30 operated, that is, the outputs of transformers 2 and3 in phase conjunction, the currents induced in the windings I3 and IIwill be in phase opposition and will annul each other, thus no currentis supplied to the transformer I3. However, as the currents inthetransformers 6 and 1 are in phase conjunction, the currents induced inthe windings 8 and 9 will also bein phase Vconjunction and power issupplied to the transformer I2, :is detected in the thermionic detectorI3, operating relay 20 and causing the operation of relay 22. Theoperation of relay 22 completes a circuit from the lower end of thewinding 9 through the right-hand make contact of relay 22 to thetransmission line, 2S. The upper end of the winding 8 is connectedthrough ground to the transmission line 2l, thus the windings 8 and 9 inaiding relationship are effectively connected across the transmissionline 23, 21. As, under (these conditions, the currents in the transform-22 to the battery 24, thus locking up the relay 22 through the breakcontact of the relay 23. Similarly, the operation of the relay 23completes a circuit from the battery 25 through the left-hand makecontact of relay 23, the center break contacts of relay 22 and theywinding of relay 23 to the battery 25 thus locking up the relay 23through the break contacts of relay 22. The operation of relay 23 thuswill cause the operation of relay 22 which will then lock up in theVoperated condition, independently of the subsequent release of therelay 29. Similarly, the operation of the rela-y 2| will cause theoperation of relay 23 which locks up independently of the subsequentrelease of relay 2|. However, as the relay22 locks up through a circuitrunning through the break springs of the relay 23, the subsequentoperationrof the relay 23 will release the relay 22, and similarly, asthe relay 23 locks up through the break spring of the relay 22, asubsequent operation of the vre- -lay 22 will release the relay23,

Thus for each change in theY phase relationship of the current suppliedto the transformers 2 and 3, there is a corresponding operation of therelay 22 or 23 and the proper windings 8 and 9. or I0 and II, areconnected to the transmission lines 26, 2l.

In Fig. 2, current from the two sources are respectively supplied to thetransformers 39 and 3I, amplified in the amplifiers 32 and 33 andsupplied to the transformers 34 and 35. The secondary windings 33 and 3lof the transformers 34 and 35 are connected in aiding relationship,While the secondary windings 38 and 39 of the transformers 34 and 35 areconnected in opposfing relationship. If the currents in the transformers34 and 35 are in phase conjunction, or as it might be termed, inparallel, the currents in the windings 33 and 3l will be in phaseconjunction and the combined output of these two windings will `besupplied through the resistance pad formed by the resistors 43, 4I and42 to the transmission line 49, 53, leading to a conventional soundreproducing channel. The currents induced in the windings 38 and 39 willbe in phase opposition and will annul each other, while the channel fromthese windings is also short-circuited by the break springs of relay 45.

If the currents in the transformers 34 and 35 are in phase opposition,or as it might be termed in push-pull, the currents induced in thewindings 36 and 37 will be in phase opposition and will substantiallyannui each other. The currents induced in the windings 33 and 39 will bein phase conjunction and will be supplied to the transformer 43,detected in the detector 44 and will operate the relay 45, placing ashort circuit across resistor 4i in the channel from the windings 33'and3l removing the short circuit normally present across resistor 4l in thechannel from the windings 38 and 39. The output of the windings 33 and39 will be supplied through the resistance pad formed by the resistors43, 41 and 48 to the transmission lines 49 and 50.

In Fig. l, the transmission channel is incomplete until the applicationof a current causes the operation of the appropriate relay to completethe channel oris completed foreither type of current, depending on theprevious operation, whereas in` Fig. 2 the transmission circuitris'normally completed for phase conjunctionor parallel current, and arelay is operated to .modiiythe channel for the transmission of phaseopposition or push-pull currents.

IntFig. 3` currents from the two sourcesare respectively supplied to thetransformers 52 and 53,1thence to the input circuit of the thermionicamplifying devices 54' and 55. The anodeof the amplier 55 is connectedthrough the transformer winding 55, resistor 58, anode battery 59, `andibiasing resistor to the cathode. The anode of the amplifier 55 isconnected through the right-hand break springs of relay 63 to thewinding 51 of the transformer, thence through the left-hand breaksprings of relay 63,'resistor 58 and anode battery 55 to the cathode'.In the un operated position of the rel-ay 63, the amplifiers `54 and 55are connected to the windings 56 and 51 in the well-known push-pullconnection. i Thus if the currents supplied by the transformers 52 and53 are in phase opposition or push-pull, the outputs of the ampliers 54and 55 will be combined by the windings 56 and 51 in the usual mannerand supplied to the transmission lines 64 `and 65. When two ampliers areoperated in push-pull and the circuit is properly balanced, it

iswell known that no alternating currents ilow in the` mid-branch fromthe center of the output transformer to the cathode, thus no alternatingvoltages will be developed acrossthe resistor 58.

11E` currents in phase conjunction, or parallel relationship, areapplied to the inputs of the amplifiers 54 and 55, the voltages inducedby the windings 55 and 51 will oppose each other and no power will besupplied to the output circuit. Howeventhe amplified current from bothamplifiers will flow through the resistor 58 in Aphase with` each otherand` will develop alternating voltages across the resistor 56 which areapplied through the capacitor 50, across the resistor 6|, detected bythe detector 62 and cause the operation of relay 63. The operation ofrelay 63 reverses the connections of the winding 51 to the amplifier 55so that the voltages induced by the windings `55 and 5'! are in phaseconjunction `and the output of both the amplifiers 54 and 55 is suppliedin the proper phase relationship to the transmission line 54, 65.However, as the outputs of the ampliers 55 and 55 are in phaseconjunction or in parallel, the alternating current continues to flow inthe resistor 58 and thus holds the relay 63 operated so longr asthecurrents in the transformers 52 and 53 are in phase conjunction. If nowcurrents in phase opposition are applied to the transformers 52 and 53,the outputs of the ampliiiers 54 and 55 will be in phase opposition, nocurrent will flow in the resistor 58 and the relay 63 will be released,thereby switching the winding 51 back to the push-pull relationship.

In Fig. 4 current from the transformers 66 and -1 61, suitably amplifiedif desired in the ampliers 58 `and 69, are supplied to the transformers16 and 1l. The secondary windings 12 and i3-are connected in aidingrelationship to the rectifying network 1S, which has substantially thesame function as the detector I5 inlFig. 1. Similarly the `secondarywindings 14 and 15 are connected in opposing relationship to therectifying network 11 which has substantially the same function as thedetector .I1 in Fig. 1. The rectifying networks 16 and 11 may be-of anydesiredtype, and as shown are full-wave bridge type rectiiers,preferablyhaving elements made up of copper, copper-oxide couples.` Theamplifying devices 18 and 19, respectively, amplify the outputs ofthei'ectifying networks 16` and 11; A double-wound differential polarizedrelay 80 has one winding connected in the `output circuit of theamplifying device 18,` and the other winding connected in the outputcircuit of the amplifying device 19. When currents in phase conjunction,or parallel relationship, are supplied to the transformers 66 and 61,the outputs of the windings 12 and 13 are combinedand rectified by thenetwork 16 causing a current lto `ow in the amplier 18 and holding thearmatures of the relay 8U outward to connect the windings 12 and 13 tothe transmission lines 8 l, 32. If currents in phase opposition, orpush-pull relationship, are appliedY to the transformers 65 and 51, theoutputs ofthe windings 14 and 15 are combined in proper'phaserelationship and supplied to the rectifying network 11 causing currentto flow in the device 119 operating the relay 8U to move the armatureofthe relay inward` Vand connect the windings 14, 15 to the transmissionlines 8l, 82. Suitable control means, such as the variable cathodebiasing resistors may lie-associated with the amplifiers 18, 1'9 tobalance the operation of the amplifiers and to adjust the anode currentsof the ampliers to values suited for the operation of the relay 8U.

In Fig. 5 currents from the two sourcesl are supplied to thetransformers 84, 85', amplified in the amplifiers 81 and respectivelysupplied to the transformers L, 85.` The secondary windings of thetransformers 68, 85 are respectively connected across adjacent arms ofthe ,balanced bridge formed by the resistors 5l), 9i, 5'2, 53.'` If thecurrents in the secondary windings of the transformers SiiV and 89 arevin phase conjunction, or parallel relationship, an alternating voltagewill be developed across the diagonal A-B of? the balanced bridge, andno voltage will be developed across the diagonal C-D of the bridge. Thevoltage developed across the diagonal A-B of the bridge is suppliedthrough the break contacts of the relay 95 to the transmission lines 9T,98 which may be connected to a regular singlechannel sound `reproducingsystem. If, however, the currents in the secondary windings of thetransformers', 8,9 are in phase opposition, or pushpull' relationship,no voltage vwill be developed across the diagonal- A-l-B of the` bridgeand an alternating''voltage will be developed across the diagonal C-D ofthe bridge. 'The voltage developed across the diagonal C--D of the`bridge is supplied through transformer 94' to the detectoi 95 and`causes the operation of relay 96. The operation of relay -96disconnects the diagonal A-B of the bridge from the transmission linesS1, 98 and connects the diagonal C-D of the bridge through the makecontacts of the relay 56 to the transmission line 91, 98, thus supplyingcurrent to the sound reproducing channel.

InV all of the gures, a preliminary stage of amplification, such as thetransformer 2 and amplifier 4 in Fig. 1, is shown in each channel. If apreampli'er is associated with each of the photoelectric cells, or ifthe output of the pick-up is suiciently great, this stage of preliminaryamplification frequently may be omitted, and thecurrent from the twosources supplied directly to the second transformers, such as thetransformers and 1 in Fig. 1. l, l

In Fig. 1, a sensitive relay 20 haviriga small number of contacts isassociated with a less sensitive relay 22 having a large number ofcontacts. It isnot always necessary that two relays should be used inthisV connection as in many cases the relay may be equipped with othercontacts similar to those shown on the relay 22 and the relay v22 thenmay be unnecessary. Similarly in Figs. 2, 3, 4,- and 5 where only asingle relay is shown, in many cases it may be advisable to use acombination of two relays as shown in Fig. 1.

Three types of push-pull recording are known to the art, and have beendesignated as type A, type B and type AB. Type A recording is shown inU. S. Patents 1,930,362, October 10, 1933, H. Konernann and 2,077,193,April 13, 1937, E. C. Wente. Type B recording is shown in U. S. Patent1,338,719, May 4, 1920, F, W. Adsit. Type AB recording is shown in U. S.Patent 2,030,428, February 11, 1936, I. M. Chambers. In type Arecording, in the absence Vof sound, both records are at some convenientpoint, preferably the center, of the recording characteristics. Thefirst half wave of the sound moves one record upward `on thecharacteristic and simultaneously moves theother record downward on thecharacteristic. The currents reproduced from such a pairv of records,under ideal conditions, have exactly the same wave shape but are opposedin phase, oscillate upward and downward about a mean value, exist at alltimes, and are, at all times, equal in amplitude. In type B recording,in the absence of sound, both records are at the bottom, or zero value,of the recording characteristics. y one record upward and then downwardover the characteristic, the second record remaining unaffected at thezero value. The second half wave of sound, which is essentially opposedin phase to the first half wave, moves the second record upward and thendownward over the recording lcharacteristic, the first record remainingunaffected at the zero value. This alternate action continues for thefollowing half waves. 'Ihe currents reproduced from such a pair ofrecords, under ideal conditions, have the wave shape of successive halfwaves of the sound, are opposed in phase, oscillate from a minimum valueto a larger vvalue then back to the minimum value, do not exist at thesame time but succeed each other, and are only approximately of the sameamplitude. In type AB recording, for small amplitudes of the sound therecord is a type A record, which for larger amplitudes of the sound,becomes a type B record. All of the reproducing systems shown in thepresent application will reproduce type A lrecordswhile the system shownyin Figs. 2 and 5 of the drawings will also reproduce type B, or typeAB, records. In the claims forming part of the present application, theexpressions phase conjunction and phase opposition are to be read asincluding not only the true, simultaneous. phase relationship of thetype A reproduction, but also the pseudosuccessive phase relationship ofthe type B and type AB reproduction.

.What is claimed is:

1V. In a sound reproducing system, in combination, a plurality ofelements for scanning a plurality of sound records, a sound reproducingcircuit,- means automatically responsive to the phase relationship ofthe currents from said scanning elements, and means controlled by saidresponsive means for connecting the outputs of said scanning elements tosaid reproducing circuit so that the currents from said elements in thereproducing circuit are in phase conjunction.

The first half wave of sound moves `2.,In a transmission system, aplurality of sources of signal currents, a circuit for utilizing thecurrents from said sources, means automatically responsive to the phaserelationship of the currents from said sources, and means controlled bysaid responsive means for supplying the'currents from both said sourcesYin phase conjunction to said utilizing circuit.

3. In a transmission system, in combination, two sources of alternatingcurrents of similar wave form, a transmission channel for each of saidsources, detector means associated with said channels, and meanscontrolled by said detector means for connecting both said channels to autilization circuit so that the currents from both said sources are inphase conjunction in said utilization circuit.

4. In a transmission circuit, in combination, two sources of alternatingcurrents of similar wave form, a transmission channel connected to eachof said sources, a transformer in each of said channels having a doublesecondary winding, means associated with said transformer for detectingthe phase relationship of vthe currents in said channels, and meansassociated with said detector means for selectively connecting thesecondary windings of said transformers to a utilization circuit so thatthe currents from both said sources 'are in phase conjunction in saidutilization circuit.

5. In a sound reproducing system, two sound records of the samealternating current, said records being in parallel or in push-pull,scanning means individual to each of said records, a transformerconnected to the output of each of said scanning means, said transformerhaving double secondary windings, detector means associated with thesecondary windings of said transformers, a single channel soundreproducing circuit, and means associated with said detector means forso connecting the secondary windings of said transformers to said soundreproducing circuit that the currents fom both of said transformers arein phase conjunction in said sound reproducing circuit.

6. In a transmission system, two sources of alternating currents ofsimilar wave form, a transformer connected to each of said sources, saidtransformer having double secondary windings, one winding of each ofsaid transformers being connected in aiding relationship and one windingof each of said'transformers being connected in opposing relationship,detector means associated with said secondary windings, and meanscontrolled by said detector means for connecting said secondary windingsto a utilization circuit in such manner that the currents from saidsecondary winding are in phase conjunction in said utilization circuit.

7. In a transmission system, two sources of alternating currents ofsimilar wave forni, a transformer connected to each of said sources,said transformers having double secondary wind ings one of the windingsof eachof said transformers being connected inaiding relationship andone ofthe windings of each of said transformers being connected inopposing relationship, detector means associated with the aidingwindings, other detector means associated with the opposing windings,and means controlled by said detector means for connecting said aidingwindings to a utilization circuit when the current from said sources arein phase conjunction and for connecting said opposing windings to saidutilization circuit when the currents from said sources are in phaseopposition.

8. In a transmission system, two sources of alternating currents ofsimilar wave form, said currents being either in phase conjunction or inphase opposition, transformer means connected to said sources tonormally transmit currents having one phase relationship to autilization circuit, means associated with said transformer means fordetecting the phase relationship of the currents from said sources andmeans controlled by said detector means for switching the connections ofsaid transformer means to said utilization circuit when the currentsfrom said sources are in the opposite phase relationships, whereby thecurrents supplied from said sources to said utilization circuit are atall times in phase conjunction.

9. In a sound reproducing system, two records of the same sound, Saidrecords being either in parallel or in push-pull, individual pick-upsfor each of said records, transformer means associated with the outputsof said pick-ups, said transformer means normally connecting the outputsof said pick-ups to a signal reproducer in parallel when one type ofrecord is being reproduced, means associated with said transformer meansfor detecting the use of a record of the other type, and meansassociated with said detector means for changing the connections of onetransformer to said reproducing circuit whereby the currents from saidpick-ups are always supplied to said reproducing channel in parallelrelationship.

10. In a sound reproducing system, two records of the same sound, saidrecords being in parallel or in push-pull, scanning devices individualto each record, a transformer in the output of each of said scanningdevices, said transformers having double secondary windings, one of thesecondary windings of each of said transformers being connected togetherin aiding relationship and one of the windings of each of saidtransformers being connected together in opposed relationship, a singlechannel sound reproducing circuit, a connection from one end of saidaiding windings and from one end of said opposing windings to one sideof said reproducing channel, a detector amplifier connected to saidaiding windings, a relay operated by the output of said detectoramplifier to connect the other end of said aiding windings to the secondside of said reproducing channel, a detector amplifier connected to saidopposed windings, a relay actuated by the output of said second detectoramplifier to connect the other end of said opposing windings to thesecond side of said reproducing channel.

11. In a transmission system, two sources of alternating currents ofsimlarwave form, said currents being in phase conjunction or in phaseopposition, a transformer connected to each of said sources, each ofsaid transformers having two secondary windings, one secondary windingof each transformer being connected together in aiding relationship, andthe other winding of each transformer being connected together inopposed relationship, said windings connected in aiding relationship andsaid windings connected in opposed relationship being connected to aload circuit, a detector amplifier connected across said opposed windingand a relay operated by the output of said detector amplifier toshort-circuit said aiding winding.

12. In a transmission system, two sources of alternating current ofsimilar wave form, said currents being infparallel relationship or inpushpull relationship, a pair of thermionic amplifying devices havingtheir input circuits respectively connected to said sources, and theiroutput circuits connected in opposed or push-pull relationship to anoutput circuit, a detector associated with the mid-branch of said outputcircuits and a relay actuated by the output of said detector to switchsaid output circuit from push-pull to parallel relationship with saidoutput circuit.

1'3. In a sound reproducing system, two records of the same sound, saidrecords being in parallel or push-pull relationship, a scanning devicefor each of said records, a pair of thermionic ampliners having theirinput circuits respectively associated with said scanning devices, asound reproducing circuit, a transformer having two primary windings anda secondary winding connected to said sound reproducing circuit, saidprimary windings being normally connected to said thermionic amplifiersin push-pull relationship, an impedance in the mid-branch of said outputcircuits, a detector energized by the voltages developed across saidimpedance, and a relay operated by the output of said detector to switchone of said windings from the push-pull connection to the parallelconnection with the other winding.

14. In a transmission system, in combination, two sources of signalcurrents, a transmission channel for each of said sources,`detectormeans associated with said channels, and means controlled by saiddetector means for connecting both said channels to a utilizationcircuit so that the currents from both said sources are in phaseconjunction in said utilization circuit.

15. In a transmission system, two sources of signal currents,transformers respectively connected to each of said sources, each ofsaid transformers having two secondary windings, one pair of secondarywindings being connected together in aiding relationship, and the otherpair of secondary windings being connected in opposing relationship,each of said pairs being connected to a common load circuit, a detectorconnected across said opposing windings, and a relay normallyshort-circuiting said opposing windings and operated by the output ofsaid detector to remove the short circuit from the opposing windings andto short-circuit the aiding windings.

WALTER A. MACNAIR.

